1. Field of the Invention
This invention relates to sacrificial cathodic protection systems and particularly to a new technique for effective and economical corrosion protection of periodically immersed structures.
Corrosion of metals in moist soil and seawater is basically due to electro-chemical reactions, involving the corroding metal and the corrosive environment or electrolyte.
It is known that cathodic polarization reduces the corrosion rate by suppressing metal dissolution and accelerating reduction reactions. Application of a cathodic potential to a metallic structure, can be effected by an external power supply (Impressed Current Cathodic Protection), or by galvanic coupling to a more anodic material (Sacrificial Cathodic Protection).
Because of its simplicity and autonomy, sacrificial cathodic protection is widely applied for the prevention of corrosion of underground structures, harbour installations, marine vessels and off-shore constructions.
2. Description of the Prior Art
Hitherto, sacrificial cathodic protection systems have been designed according to potential and current density criteria, depending essentially on the structure to be protected, the type of anode material, the corrosive environment and the projected service life.
In practice, allowances are made for anode consumption and deterioration in time, to ensure that the criteria for corrosion protection are met during the entire design life of a sacrificial protection system.
For applications to structures subject to periodic immersion, e.g. structures subject to tide or groundwater level fluctuations, ballast water tanks, and cargo-ballast tanks in seagoing vessels, additional protection is required to ensure fast polarization during the successive immersion periods.
Cathodic protection systems, thus designed have not met with unqualified success. In order to provide cathodic protection over an extended period of time, the sacrificial anodes used must be large and numerous enough to provide adquate current flow up until their normal time of replacement. This inevitably results in either overdimensioning of the system leading to anode wastage and problems associated with overprotection (e.g. cathodic disbonding of coating systems, hydrogen evolution, undesired build up of calcareous deposits), or underdimensioning, penalized by an increased corrosion rate, or the need for more frequent anode replacements. These disadvantages can be offset by regulating the anode current output, so that overprotection and anode wastage is avoided.
There is a U.S. Pat. No. 3,360,452, issued Dec. 26, 1967, to F. E. McNulty, which is concerned with a cathodic protection system, basically like to the type of system which this application involves.
A power transistor is used to regulate the anode current output, in order to maintain the potential of the structure at a predetermined level.
The effectiveness of such system is however significantly reduced by the offset voltage drop accross the transistor emittor-collector junctions.
It is an object of the present invention to provide improved means for regulating the current output of a sacrificial anode, or of a group of anodes. The superior characteristics of fieldeffect or MOS (metal oxide silicon) semi-conductor switches are used. These switches have no offset in the ON condition, allowing optimized use of the anode driving power. This is of particular importance in applications where fast restoration of cathodic polarization is required.
Consequently, it is another object of this invention to provide a sacrificial anode type of cathodic protection, which has superior ability to provide fast restoration of protective potentials, e.g. in the case where the protected structures are periodically immersed. It is a further object of this invention to eliminate any need for an external power supply for operation of the current regulating device or auxiliary circuitry.
In addition, it provides a substantial saving in anode consumption, so that the life of the sacrificial anode or anodes employed is greatly increased.